A radioimmunoassay for human prolactin has been developed and is being used to measure serum prolactin levels in patients with breast cancer and in patients with galactorrhea and gynecomastia. The effects of various therapeutic modalities in patients with breast cancer, such as endocrine ablations or administration of hormones or chemotherapeutic agents are being studied for their effects on prolactin secretion. Drugs which may inhibit the secretion of prolactin under study for their possible therapeutic usefulness. Prolactin receptors in breast cancers are being measured for possible predictive value of hormone responsiveness.